Robotic observations of high wintertime carbon export in California coastal waters

Abstract

Abstract. Biologically mediated particulate organic and inorganic carbon (POC and PIC) export from surface waters is the principal determinant of the vertical oceanic distribution of pH and dissolved inorganic carbon and thus sets the conditions for air sea exchange of CO2; exported organic matter also provides the energy fuelling communities in the mesopelagic zone. However, observations are temporally and spatially sparse. Here we report first hourly-resolved optically-quantified POC and PIC sedimentation rate time series from autonomous Lagrangian Carbon Flux Explorers (CFEs), which monitor particle flux using imaging at depths below 140 m in the Santa Cruz Basin, CA in May 2012, and in January and March 2013. Highest POC vertical flux (~100–240 mmol C m−2 d−1) occurred in January, when most settling material was mm to cm-sized aggregates, but when surface biomass was low; fluxes were ~18 and 6 mmol C m−2 d−1, respectively in March and May, under high surface biomass conditions. An unexpected discovery was that January 2013 fluxes measured by CFE were 20 times higher than simultaneously deployed surface-tethered sediment traps and which multiple lines of evidence indicate strong under sampling of aggregates larger than 1 mm. Furthermore, the Jan 2013 CFE fluxes were about 10 times higher compared to highest previous nearby multi year sediment trap observations. The strength of carbon export in biologically dynamic California coastal waters is likely underestimated by a factor of between 3 and 20.